As is known in the art, the “compression ratio” of an internal combustion engine is defined as the ratio of the cylinder volume when the piston is at bottom-dead-center (BDC) to the cylinder volume when the piston is at top-dead-center (TDC)—generally, the higher the compression ratio, the higher the thermal efficiency and fuel economy of the internal combustion engine. Unfortunately, compression ratios are limited by the availability of high-octane fuels needed to prevent combustion detonation or knock at high engine loads, and therefore a compression ratio is selected to operate on available fuels, and avoid knock. So-called “variable compression ratio” internal combustion engines have been developed, for example, having higher compression ratios during low load conditions and lower compression ratios during high load conditions.
In an engine with a variable compression ratio mechanism, the engine compression ratio can be selected to achieve the best fuel economy of a vehicle. However, drivability and engine knock issues may occur by changing engine compression ratio while driving a vehicle in different environmental conditions. To ensure the switching of compression ratio happens with minimum knock and as smooth as possible at every possible real-world driving condition, not only must the engine operating conditions be taken into consideration but also environmental conditions have to be taken into considered in the compression ratio selection. The problem is how to take into account those factors so as to select appropriate engine compression ratio to obtain optimum fuel economy without sacrificing drivability.